Electroluminescent (EL) devices such as Organic Light Emitting Diodes (OLEDs) are a promising technology for flat-panel displays and lamps or illumination sources. EL devices can be formed as large, solid state devices that provide uniform light output over larger areas with a high efficiency and excellent color rendering. Further, these devices are thin, consume relatively small amounts of materials, and do not include materials that are known to be harmful to the environment. Each of these attributes is highly desirable for a display or lamp.
EL displays are typically passive- or active-matrix structures with EL emitters arranged in a two-dimensional array. Large area coatable EL lamps, such as OLED lamps, can be formed to include multiple OLEDs or other EL light-emitting elements on a single substrate wherein these OLEDs are connected in series to create a high voltage lamp. Groups of series-connected EL emitters can be themselves connected in parallel, the EL emitters being laid out in a two-dimensional array.
In lamps using these serial connections, the individual serially-connected EL elements are typically small, as several EL elements are connected in series to form high voltage lamps that support electrical potentials near the electrical potentials used in the power distribution infrastructure. Further, because a short in an EL element will dim, if not disable, an entire EL element, it is desirable to provide small EL elements to avoid large dim or dark spots within the lamp due to shorts. However, shorts can occur over the life of a lamp. Similarly, individual EL emitters in a lamp or EL display can dim over time as they are used, even if no shorts occur. There is a need, therefore, for ways to detect dimming and shorts over the life of an EL device.
Ashdown et al., in U.S. Pat. No. 7,573,210 and U.S. Pat. No. 7,573,209, describe schemes for feedback and control of a luminaire with one or more LED lamps, including light sensors for detecting the light emitted by the lamps and a control system for adjusting the current to one or more of the lamps to maintain the light output at a desired value. However, these schemes do not recognize the problems of short detection, reporting to a central monitoring system such as a building management system, or placing the light sensors so that they do not obstruct the light reaching a user.
Muthu et al., in U.S. Patent Application Publication No. 2003/0230991, describe an LED backlight unit (BLU) including photodiodes for measuring the luminosity of the light in a light guide and a control circuit for maintaining the color and luminosity of the BLU. However, this scheme affixes the photodiodes directly to the light guide, making the BLU an expensive, integrated unit that must be entirely replaced if any part fails. Furthermore, this scheme is adapted to an edge-illuminated light guide that has the same luminosity and color throughout and cannot detect the spatial locations of failures of individual emitters, such as are found in EL devices illuminated by EL emitters located on the face of a substrate rather than the edge.
There is a continuing need, therefore, to detect variations and failures in the light output of a face-illuminated EL device without obstructing the light path to the user or making the measurement electronics part of an expensive, difficult-to-replace component.